Automotive ventilation systems, often called HVAC systems (heating, ventilation and air conditioning systems) typically include a large, hollow housing located just behind the instrument panel of the vehicle interior. The housing includes the two heat exchangers, the cold evaporator core and hot heater core, as well as the various valves and doors that route intake air through or around the two to create outlet air of a desired temperature. Outlet openings in the housing and instrument panel direct the air up at the windshield, forwardly at the occupant, or downwardly at an occupants' feet. These openings are covered or uncovered by a suitable valve, as selected by the vehicle occupant. Typically, the outlet openings, in descending order of height, are called defrost, air conditioning, and heat, although air of any temperature may be directed through any of them.
Different valves have been used to selectively cover and uncover the outlet openings, depending on the shape and orientation of the openings. In designs where the perimeters of the outlet openings are disposed in flat planes, it is relatively easy to use a so called film valve. In a film valve a belt or sheet of flexible film is stretched over rollers into one or more straight runs, which reside just behind the perimeter edges of the outlet opening or openings. As the film belt rolls around the rollers, back and forth, windows and solid portions in the belt alternately register with the outlet openings, covering and uncovering them. One drawback is the conflict between tightness of the seal and relative rubbing. If the surface of the belt is held flush against the interior surface of the perimeter edges of the outlet opening (usually against a foam seal that surrounds the opening), there is a frictional rubbing that retards the motion of the belt. If it is not held flush against the foam seal, there is not a complete blockage of air. Another known outlet valve is a rotary valve. With a rotary valve, the various outlet openings have perimeter edges disposed in semi cylindrical arcs and a semi cylindrical solid door swings up and down inside the housing to cover and uncover the openings. Seal rubbing is also a problem with a rotary valve. Another drawback with a solid rotary valve is that it must be designed with overtravel, that is, it must swing completely past the opening perimeter in order to completely uncover it, which detracts from compactness.
Designs have been proposed in which a film belt is used to cover and uncover outlet openings which are curved, rather than flat. This requires additional structure since the natural orientation for a belt run is flat, the shape that it naturally takes on when stretched over a pair of rollers. As shown in U.S. Pat. No. 5,105,730, the belt must be stretched over a rigid, concave curved supporting frame keep it in the necessary curved shape. This worsens the rubbing friction problem, since the film belt is now sandwiched between both the outlet opening's perimeter edge seal and the added supporting frame. An even earlier design, proposed for a domestic room air conditioner, appears to eliminate or reduce the rubbing by running a continuous, solid loop of film back and forth along a curved, rigid, but concave supporting frame. The frame is located behind a pair of side by side outlet openings, and the center of the belt loop is pinned to the frame, half way between the openings. Two rollers are spring biased apart inside the loop so as to keep it taught. The rollers roll back and forth to shift each half of the loop back and forth cover or uncover one or the other of the side by side openings. This design, while it largely eliminates rubbing, would not be adaptable to the automotive HVAC environment. The supporting frame that gives the belt run its shape must be concave or, at best, flat, and could not be convexly curved. In addition, the potential motion of the film belt back and forth is limited by the fact that it has to be pinned in the center to the frame. The rigid supporting frame needed in either design is, of course, an additional part that increases expense and takes up space within the housing.